The present invention relates to the separation of sulfur compounds from a hydrocarbon mixture using an ionic membrane.
Sulfur compounds are impurities in gasoline that compromise vehicle emission controls by poisoning the catalytic converter. In an effort to further decrease emissions, the U.S. government has recently proposed a nationwide reduction of sulfur in gasoline from current levels at 300-1000 ppm to an average of 30 ppm (Federal Register, 64(92), May 13, 1999). Gasoline producers, both domestic and foreign, selling fuel in the U.S. would be expected to comply by the year 2004.
Presently, the conventional process for reducing sulfur content in gasoline involves hydrotreating in which sulfur compounds are converted to volatile hydrogen sulfide and other organics. This energy intensive process, requiring elevated temperature and pressure, is expensive for obtaining the proposed lowered sulfur levels. Alternative processes with more efficient sulfur-reducing technology are needed to maintain progress toward cleaner burning fuels.
The use of membrane separation technology, in which select compounds or types of compounds can be separated from an organic mixture by permeation through a membrane, has been reasonably well developed. Separation processes that incorporate membranes present an attractive option for large-scale purification of petroleum fractions because of their inherent simplicity, versatility, and low energy consumption.
Typically, membrane separation processes rely on the affinity of a specific compound or class of compounds for the membrane. In this way, the components of a mixture with specific affinity for the membrane will selectively sorb onto the membrane. The sorbed compounds diffuse, or permeate, through the membrane and are removed on the opposite side. Continual withdrawal of permeated compounds from the membrane maintains the driving force for the separation process. Removal of permeated compounds is usually achieved by pervaporation or perstraction methods. Pervaporation employs a vacuum on the permeate side of the membrane, removing the permeated compounds in gaseous form, while perstraction employs a liquid sweep stream, continually washing away permeate.
The chemical properties of the membrane dictate the type of compound that has affinity for it. Some types of membranes are composed of charged chemical groups and are, therefore, considered ionic in character. An example of an ionic membrane is Nafion(copyright) (available from DuPont, of Wilmington, Del.) which is a polymer of perfluorosulfonic acid that has been used principally in the dehydration of liquid organic mixtures as described in U.S. Pat. No. 4,846,977. Only few examples exist for the use of Nafion(copyright) in separating organic compounds. U.S. Pat. No. 4,798,764 describes the separation of methanol from dimethyl carbonate or methyl t-butyl ether. The use of Nafion(copyright) membranes in the separation of mixtures of styrene and ethylbenzene has also been reported (Cabasso, Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 313). Additionally, U.S. Pat. No. 5,498,823 reports the enhanced separation of unsaturated organic compounds using silver ion-exchanged Nafion(copyright) membranes. A related ionic membrane composed of sulfonated polysulfone has been also used for the separation of aromatics and non-aromatics as disclosed in U.S. Pat. No. 5,055,631. To date, the use of ionic membranes, such as Nafion(copyright), in the separation of sulfur compounds from liquid organic mixtures has not been reported. All of the above mentioned patents and article are incorporated herein by reference.
The proposed mandate for lowered sulfur levels in gasoline has made it imperative to improve or replace existing methods for desulfurization of petroleum fractions. A more cost-effective method for reducing sulfur content in petroleum fractions is a primary goal of the oil refining industry.
This invention relates to the separation of sulfur compounds from a hydrocarbon mixture using an ionic membrane. The membrane is composed of a material with acidic or charged chemical groups such as the perfluorosulfonic acid groups in Nafion(copyright)-type membranes. Preferred membranes permeate sulfur compounds over hydrocarbons such that a hydrocarbon mixture containing sulfur compounds is separated, by the membrane, into a sulfur-rich fraction, i.e., sulfur-rich permeate, and a sulfur-lean fraction, i.e., sulfur-lean retentate.
The present invention provides a method of separating sulfur compounds from a hydrocarbon mixture using an ionic membrane, said hydrocarbon mixture containing at least one sulfur compound and hydrocarbons, comprising the steps of:
(a) contacting said hydrocarbon mixture with said membrane;
(b) selectively permeating said sulfur compounds of said hydrocarbon mixture through said membrane forming sulfur-rich permeate and sulfur-lean retentate; and
(c) retrieving said sulfur-rich permeate and said sulfur-lean retentate.